The invention relates generally to optical communications such as a system and method for optical communications with data rates of at least 10 Gbit/s.
Many optical communication systems with data rates of at least 10 Gbit/s are based on a return-to-zero (RZ) format for data transmission. Such known optical communication systems typically relate to generating an optimal pulse shape or to evaluating the pulse shape once generated.
For example, adaptive pulse shaping in free space and in the 850 nm spectral range has been disclosed in D. Yelin et al., “Adaptive femtosecond pulse compression,” Opt.Letters, 1997, v.22, #23, pp. 1793-1795. The input pulse arrives from a comb generator (in this case, a mode-locked laser), and is spatially separated into its spectral component by a grating. A lens maps each wavelength group onto a separate pixel of a computer-controlled phase modulator. A second lens and grating recombine the components back together to form a controlled output pulse. A doubler crystal samples the output pulse. This crystal produces pulses that are higher the shorter the pulse is. By using an appropriate algorithm, a computer can adapt the phases of the different spectral components of the input pulses so that for each type of input pulse, the shortest pulse possible should be produced by the device. The shortest pulse is considered as the most optimal for the communication system.
An alternative but analogous method was disclosed in K. Kitayama et al. “Optical pulse train synthesis of arbitrary waveform using weight/phase-programmable 32-tapped delay line waveguide filter,” Proceedings of OFC-2001, paper WY3-1. Similar to the Yelin system, the pulse source in Kitayama is a comb generator, but the pulse shaping is done through a parallel series of delay lines and attenuators.
These known devices, however, suffer several shortcomings. For example, these known devices are quite complex from the technical point of view. In addition, because these known devices typically relate to generating an optimal pulse shape or to evaluating the pulse shape once generated, such devices are bulky, expensive and not appropriate for use in commercial systems.
Moreover, in real optical communication system, either terrestrial or undersea, the fiber conditions and multiple component operations change in time. Therefore, the optimal pulse shape is different for every particular time interval. The best performance of the pulse generator or pulse shaper should include a closed loop to adaptively correct the changing conditions. An adaptive approach for the pulse shaping in fiber communication has been developed by a number of research groups (see, for example, F. G. Omenetto, M. D. Moores, B. P. Luce, D. H. Reitze and A. J. Taylor “Femtosecond pulse delivery through single-mode optical fiber with adaptive pulse shaping,” Proceedings CLEO' 2001, pp. 234-235). Indeed, such an approach can provide a mechanism to overcome multiple limitations associated with nonlinear effects and provides an opportunity to synthesize pulses that are self-correcting for higher order nonlinear effects when being launched in the fiber.
As discussed below, in the present invention, a device is described that can be implemented in real RZ communication systems and can provide a number of advantages from the point of view of chromatic dispersion reduction and nonlinear effects mitigation. This results in an improvement of the communication link figure of merit: cost/(capacity*distance). The described device provides new technical solutions in the pulse formation and in the pulse shape evaluation together with adaptive shaping in time.